The production of wafers, especially tablets, based on zeolites and binders is already known. Thus, according to JP-A-61 15 5216, zeolite tablets are produced by mixing a zeolite, a binder and a lubricant and extruding the mixture. They are formed as tablets with the same dimensions in all directions.
JP-A-56063818 discloses production of zeolite tablets for use as gas adsorbents. Powdered zeolite dried at 105 to 110° C. are mixed with 8.1% by weight bentonite power and are kneaded with a 4% aqueous urea solution. The mixture is tabletted, dried and calcined at 510° C. The increase in the compressive strength is dictated by the urea content.
JP-A-55 16 5144 discloses kneading zeolite powder for refrigerant aggregates in powder form with bentonite and water, extruding the mixture and forming round particles with a diameter from 0.8 to 10 mm.
According to JP-A-55 10 4913, zeolite in the Na form is mixed with 25% by weight clay, kneaded with water, extruded, calcined at 650° C., immersed in a calcium chloride solution, washed, dried at 110° C. and activated at 400° C. The tablets are used as desiccants.
According to JP-A-603 2572, zeolite powder is mixed with kaolin and Na (or NH4)-hydroxymethyl cellulose, shaped, dried and calcined at 650° C., in order to increase the strength of the zeolite tablets.
According to JP-A-21 44 121, deodorants are prepared by extruding zeolite powders or grains with calcium chloride or bentonite and water. The mixture is then tabletted and the tablets calcined.
According to JP-A-63 218 234, desiccants are produced by extruding a mixture of microporous particles (for example, gypsum, cement, ceramic powder) and an inorganic or organic filler, such as CaCl2, LiCl, bentonite, zeolites, PVA or other water-soluble polymers. The mixture is tabletted and then hardened.
According to JP-A-60 132 643, zeolite tablets are produced as desiccants using 20% sepiolite as the binder. The mixture is kneaded with water, tabletted, dried at 150° C. and calcined at 550° C. The tablets have an improved drying effect compared to bentonite tablets.
The tablets produced according to the prior art are unsuited for use under spatially restricted conditions and under mechanical stress since they are too thick and too heavy and in terms of mass and surface, have overly low sorption capability for toxic gases and vapors. With the processes and mixtures according to the prior art, overly brittle wafers are obtained which crumble especially after calcining.
It is known that electroluminescent devices work properly over a longer time only when a desiccant is present. This can be attributed to the sensitivity of the electrodes, especially of the cathodes, for example, to moisture (the cathodes consist of Ca or Mg alloys). Therefore these devices are sealed as well as possible under a protective gas.
In EP 500 382 A2 the use of a moisture absorber in an electroluminescent device is described. The desiccant in the form of a powder or beads is applied to a black silicone resin coating. According to the preferred embodiment the desiccant is placed in a gas-permeable bag.
U.S. Pat. No. 5,882,761 describes the use of a desiccant in an electroluminescent device. BaO is used as the preferred desiccant. See also U.S. Pat. No. 5,591,379 which discloses a coating for use with microelectronic devices comprising a desiccant powder blended with a binder.
The sorbents known from the aforementioned publications have the disadvantage that they can sorb only water vapor. An attack on the cathode can also be triggered by other gases which, in addition to water, form when the epoxy resin which is used for sealing sets (ammonia, volatile amines). In addition, the action of oxygen also can lead to failure of the luminescent components (oxidation of the cathode).
The object of this invention is to produce disk-shaped wafers based on an inorganic sorbent and an inorganic binder, with a very low thickness (less than 700 microns) which in spite of their low thickness have high strength and thus can be installed, especially in electronic components, in which only limited space is available and which can be exposed to vibrations (for example, electronic display devices in motor vehicles and mobile telephones).
This object is achieved by preparing a disk-shaped wafer based on at least one inorganic sorbent and at least one binder, with a thickness of less than 700 microns, which can be obtained by pressing a mixture of or a mixture containing an inorganic sorbent, about 20 to 60% by weight of the binder and about 10 to 15% by weight water (relative to the overall mixture) at a pressure of at least 70 MPa; and calcining the resulting green wafer at temperatures of at least about 500° C. until the water content is substantially removed.
The wafers as claimed in the invention have high strength, low brittleness, high sorption rate and high sorption capacity at low mass. They exhibit low thermal expansion, no wear and can be easily colored by adding pigments during manufacture.